Today while patching a leak, I noticed my tube is for 25-32 tyres, and my tyre is a 35. Is that really an issue? I wasn't aware of my mismatch, and I have driven quite a lot on these wheels with no noticeable problems. Now i'm just wondering if I should keep using this tube (it has two patches now).

Edit: also, the reason I was patching it because it lost maybe half of the air in ~48 hours, so not a massive hole but still a hole. I suspect it was from a piece of glass, which I thought i saw on my tyre but now seems to have come out (I had to get to work so I had no time to patch it immediately when i noticed the leak, it kept air in well enough for my commute).

I don't think that the width is an issue, because when you pump it up, it will expand

It's not optimal, but it'll work fine. I wouldn't worry about it. Tubes for wide tires in a narrow tire are more problematic, in my experience.

It expands, but also thins out. I really don't think it's going a be a big problem in this exact case, since it's only slightly undersized, but there is a limit.

As an example, many years ago I was in a small group ride with 32mm tires and had flat. My spare was also flat, and the closest tube another rider had was for 25mm max. It worked fine for about a mile and then loudly popped.

Because the rubber is being stretched, a puncture can tear open like a balloon being pinpricked. A puncture in a fitting inner tube leaks slower.

You can slow down air leakage further by sticky-taping the wheel's spoke holes airtight. It helps by letting you ride to a more comfortable place for patching after noticing you have a leak.

Or explode! If it hasn't then it's probably fine.