For the past week or 2 now I have my ilevel either not showing for people or it always being 435 or 411 when i'm a 465. No one accepts me into mythics and I can't seem to figure out how to fix it. tried uninstalling and reinstalling the addon, also tried to clear the cache and purge settings etc. Still doesn't fix my issue.

There is a difference between "in the bags" and "equipped".
Some times an addon makes wrong assumptions about forged items, 2handers, and special items like the heart of azeroth.
But: the real value that the ingame api calculates should be what people make their decisions on.
I doubt the majority out there relies on just SimpleILevel.
Therefore be precise which part of your ui , other peoples ui show which value.
If in doubt a few screenshots will make things clearer.

i have the same problem. my GS ingame is 472 and the addon says 440. the problem is also, that my mates see this GS too

I'm not sure why this is happening as I am unable to reproduce it. This may be a duplicate of #9. SIL does not look at bags, only what is equipped, the default UI will show what is in your bags and equipped when you mouseover.

Only way I have fixed this issue is using the version that was previous to the latest and reinstalling that. Seemed to have solved it