I recently removed the group gasket and shower screen from my E61 grouphead for the first time ever so I can give it a nice cleaning.

But when I reinserted the screen and gasket into the grouphead, I notice that my portafilter is SIGNIFICANTLY tighter whenever I need to twist it into the grouphead.

Is this common?

I thought it would fix itself over time, but it's been a week and it is still very tight. There are absolutely zero issues with making an espresso (no leaks) as the gasket and screen have been reinserted correctly.

Have I missed something?

Are you sure that 1 - There was no residue on the group that got behind the gasket, and 2 - that you did not put the gasket in upside down..? Remove it again. wet the gasket with water, replace on screen, wet again, and reinsert.