Hate to tell you this...but yes, apparently they do sell out. Checking tomorrow's trains, the 11:07 is already sold out. So is the 13:07.
(This would be the train to Brussels; the Belgian train from there to Bruges can't sell out, not reserved, though if it's full you'll have to stand.) The next train for tomorrow not sold out is at 16:07 - and who knows if that one will.
So maybe you really ought to just book the 13:07 now and eat the two hours, instead of having to pay a few bucks more for maybe the 16:07, if it's not sold out.
As much as I dislike buses, FYI there's a direct FlixBus bus from CDG to Bruges at 13:45 (duration:3:45 hours) for only $17.99 USD (vs 86.30 euros to book NOW the 13:07 train with connection in Brussels). I'd probably book that bus either way as insurance. Then see what is available for trains when you eventually arrive and if one is available and you want to take it instead of the bus, take it - maybe an extra $100 USD or more for the comfort of a train. Worth it? Hmm - probably not to me.
I've taken a FlixBus once, from Ljubljana, Slovenia to Trieste, Italy. It was OK for a bus. Free WiFi. I'm frugal, so this honestly is what I'd probably do (or book a later train much earlier, like 3 months out, obviously way too late by 1-2 weeks from arrival).
FlixBus has an app too FYI.