Let's see what you are seeing. If you are talking about filling between two separate pieces of wood, filler is not recommended in my opinion. What climate are you in? How were the window installed? Large swing in relative humidity via a poorly sealed connection to the home, will generate larger movement between the trim sections.

We're in Maryland, near DC. So hot and humid in the summer and usually not too cold in the winter. Don't know how to answer your question about how the windows were installed. Remodelers installed them. Here is a photo that shows the problem.

So, picture frame moulding install flush with the jambs and stool? This is why trim should be installed with a slight (1/8") reveal. Filler/ caulk/ whatever...will be a battle without end.

How they were "installed" pertains to how they were set. Full tear outs with flanges, full tear outs, etc.

Was this a new addition to the home? The large temp swings we are having right now can acerbate this issue and if the window are poorly sealed to the shell of the home, the humidity fluctuations will do this.

Yes, all of the affected windows are in both of two additions that were done more than 10 years ago, a sunroom on the back of the house and a partial second story on top. Not replacements. Whether they are well or poorly sealed, I guess I can't say for sure, other than to say that our heating and cooling bills are not out of the ordinary.

Only want to determine would be to pull the trim. That's a bit like cutting off your nose despite your face at this point. I would remove the old sealant and apply a good sealing in place of it. Repaint.

Sorry, I don't understand. What sealant?

There is likely caulk between the casing trim and jamb extensions. Those gaps are not "normally" covered with paint. If that's all that was used, that is likely part of your issue.