Help needed! Bitter aftertaste!???!?

i don get it..

the same recipe I been using over n over again which is

bread flour 11.7-12.4% protein

1.222kg

sugar

165gm

salt

15gm

milk powder

21gm

yeast

18gm

egg

3nos

water

663ml

butter

90gm

all these time I make my tuna filling/sausage rolls with this recipe so far no problem .

then one batch turn out to have a slight or noticeable bitter aftertaste ? i used the same ingredients n brand for all nothing changed ..why suddenly this batch got a bitter aftertaste ?

where does this bitter aftertaste came from if cant be stail already is just mere hours out from the oven .

the only thing different that I can think off is..I made the dough wrap sausage with it..n chuck into the freezer to keep.. after 1 week or so.took it and put in the fridge overnight to bake the next day.it rise nicely after 3-4 hour n bake..came out just like how it should  big fat puffy oval sausage buns

but how is freezing it n bake later gave the bitter taste or is not the freezing issue at all?

Or maybe now that it comes to mine..

i pack each bread rolls in a small plastic bag and lightly sealing it..still a bit warm and can see air vapour on the bag later ..but does this made my bread have a bitter aftertaste or not?

just looking at all the possibility ..