Is it claimed that methadone HCl is less stable in solution, in the presence of raspberries syrup?

This is a very difficult question to answer. An answer to this question is not to be found in the literature; Both syrups are very complex mixtures and which researcher will be interested in this.  

Without speaking about the difference of ingredients between the natural and the artificial syrup, a possible difference in pH is still mentioned.

Now in the presence of citric acid as foreseen in the composition of the TMF, I do not think that the pH can be much more acidic. All you can do is check if there is a noticeable difference in pH measurable.  

I would also like to remind you of the recommended composition published in the TMF. This makes use of banana aroma, which I believe is a raw material reimbursable by the RIZIV.  

orange peel or orange blossom as aroma for methadone. Lemon or   lime spiritus 2%