I would be interested to understand , where this happens and how it can happen and if there are suggestions for preventing it.

I am interested to know about this too.

Here is a 2017 open source article exploring this concern. I haven't seen anything more recent but would be interested to learn more. It specifically references two reports of unintended use, one from a UNICEF 2013 global review (reports of sharing amongst siblings) and a study out of Ethiopia on perceptions of caregivers beyond therapeutic use as intended. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5409604/ 

Abstract:

"The development of ready-to-use therapeutic food (RUTF) for the treatment of uncomplicated cases of severe acute malnutrition in young children from 6 months to 5 years old has greatly improved survival through the ability to treat large numbers of malnourished children in the community setting rather than at health facilities during emergencies. This success has led to a surge in demand for RUTF in low income countries that are frequently food insecure due to environmental factors such as cyclical drought. Worldwide production capacity for the supply of RUTF has increased dramatically through the expansion and development of new manufacturing facilities in both low and high income countries, and new business ventures dedicated to ready-to-use foods have emerged not only for emergencies, but increasingly, for supplementing caloric intake of pregnant women and young children not experiencing acute undernutrition. Due to the lack of evidence on the long term health impact these products may have, in the midst of global nutrition transitions toward obesity and metabolic dysfunction, the increased use of manufactured, commercial products for treatment and prevention of undernutrition is of great concern. Using a framework built on the life course health development perspective, the current research presents several drawbacks and limitations of RUTF for nutrition of mothers and young children, especially in non-emergency situations. Recommendations follow for potential strategies to limit the use of these products to the treatment of acute undernutrition only, study the longer term health impacts of RUTF, prevent conflict of interests arising for social enterprises, and where possible, ensure that whole foods are supported for life-long health and nutrition, as well as environmental sustainability.

Bazzano AN, Potts KS, Bazzano LA, Mason JB. The Life Course Implications of Ready to Use Therapeutic Food for Children in Low-Income Countries. Int J Environ Res Public Health. 2017 Apr 11;14(4):403. doi: 10.3390/ijerph14040403. PMID: 28398257; PMCID: PMC5409604.

I think that Bazzano paper was slightly contriversial. Since then, there have been studies that show no evidence that RUTF leads to excess fat gain - it leads to gain in lean mass (muscle) rather than fat - see this study https://pubmed.ncbi.nlm.nih.gov/33534818/ and this study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5593178/ - these studies are both from children with MAM

I know a systematic review on this topic was started, but I don't know what happened to it - the plan is registered here: https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=254400

On the point of HEALTHY children consuming RUTF, Im not aware of any studies that have looked at this since it is not recommended that healthy children should consume this therapuetic product

As pointed out by Natasha, fears that treatment of SAM with RUTF may lead to an excess fat mass gain are regularly expressed without hard evidence. The rationale is that RUTF provides 45% to 60% of its energy from fat and it is assumed that fat foods lead to fat accumulation. Real data rather suggest this fear is rather unfounded. In addition to already quoted articles in the previous post, please see this article in SAM children:

Binns P, Myatt M. Does treatment of short or stunted children aged 6-59 months for severe acute malnutrition using ready to use therapeutic food make them overweight? Data from Malawi. Arch Public Health. 2018 Dec 13;76:78.

It has never been shown clearly that high fat foods lead to fat accumulation in infants and young children. As a reminder, breast milk also provides about half of its energy from fat and breastfeeding is not regarded as predisposing to obesity.

Having said that, RUTF consumption of RUTF by healthy children should be discouraged: RUTF is an expensive scarce commodity designed for SAM children, and these children should receive it in priority. Also, its high sugar content may be a concern if eaten for a long time.

Just to add to Natasha and Andre's comments, this study also showed that kids with MAM also gained predominately fat-free mass when supplemented with rusf. 

https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1002387

Fabiansen C, Yaméogo CW, Iuel-Brockdorf A-S, Cichon B, Rytter MJH, Kurpad A, et al. (2017) Effectiveness of food supplements in increasing fat-free tissue accretion in children with moderate acute malnutrition: A randomised 2 × 2 × 3 factorial trial in Burkina Faso. PLoS Med 14(9): e1002387. doi:10.1371/journal.pmed.1002387