....
My question is, at different stages does Chess.com select opponents that force you away from the openings/defense that normal favour or have I just hit some weird patch?
No, opening choices have no bearing on pairings.
As Martin_Stahl said, no, this is not the case. However, I have found that typically opponents within certain ELO brackets tend to play rather similar openings and so you may find that as you rapidly climb or fall that you face a 'snapshot' of that ELO, facing a much higher percentage of the same unusual openings than you might be used to.
This is going to sound weird and maybe a little conspiracy theorist, but....
I am returning to chess after many years and thoroughly enjoying it. I'm not a great player but working on it. Here's the chess.com journey....
Start well enough,
Rating plummets
Start studying one opening (Ponziani) and one defense (Scandinavian).
Over the course of a few weeks I move from low 700s to low 900s. With a huge amount of opponents following the book moves for both. I do mean a huge amount. I progress with those two opening/defense. I hit something like 920 and almost every opponent is now playing anything but those. I've had everything but them. Suddenly I drop 120 points in about 20 games.
My question is, at different stages does Chess.com select opponents that force you away from the openings/defense that normal favour or have I just hit some weird patch?