SOLUTION/CAUSE:
I eventually found out that my parameters had the data type boolean but in fact I use them as integers.
So it appears the reason why DA altered a -1 into a 1 is because he interpretes it as true as soon as it is not 0
So this is a bug in my DA schema but I feel DA could have helped a bit.
The schema modeler generated the parameters and boolean data type entirely on its own based on the SQL below.
I believe it would be better if it did not assign a type yet.
An alternative would be to have DA do data type checking on the parameters and throw an error when I pass an integer to a boolean parameter.
I have posted my issue because others might run into the same issue and/or the people at RemObjects might add data type checking which would have helped me in the long search after this bug.
SELECT prd_primkey,prd_cnk,pnm_naam,prd_korting,prd_lok_primkey,lok_lokatienaam,
prd_automatischbestellen,prd_lev_primkey,prd_minvoorraad,prd_maxvoorraad,
prd_aantaltebestellen,prd_Wetgeving,prd_Bereiding,prd_BewaarTemperatuur,
prd_eigenaankoopprijs,prd_publieksprijs,prd_eigenprijs,prd_richtprijs,
prd_stock,prd_heeftvervaldatum,prd_atc_code,
prd_terugnamestatus,prd_terugnamevoorverval,prd_terugnamenaverval,prd_isleverbaar,
pnm_nederlands,pnm_frans,
prg_primkey,prg_rel_primkey, prd_sbs_Primkey
FROM vProductDetailLightML
LEFT JOIN productgroep ON prd_primkey=prg_prd_primkey
WHERE {Where}
AND ( (pnm_nederlands = :dynNL) OR ( :dynNL =-1) )
AND ( (pnm_frans = :dynFR) OR ( :dynFR =-1) )