Hello
I'm going to buy a new 27.5" hardtail, because the frame of my old Trek 3700 broke. I'm 1.90m tall(6'3") and have a inseam length of about 90cm (2'11"). Recommended MTB frame sizes for my inseam&height are between 20-22". The problem with the bike I'm going to buy is that it's 19.5" .
However, my old 26" Trek was 18", but I was riding it very comfortable and without discomfort, especially if I raised the saddle a little. I also loved how I could take corners fast and stable when sitting on the saddle at its lowest setting. Taking this into account, would the 19.5" frame of the new bike not be such a big problem ?

In my opinion, no problem. Some riders like a bike that is one frame size smaller than "recommended". It's easier to throw the bike around on technical trails. Then again, I'm biased because I'm 5'9" and ride a small, 15.5" Stumpy hardtail and a small size Stumpy dual sus, not the "recommended" medium, 17/18 inch frames.

A 18" frame is tiny for someone 6'3". Perhaps you haven't ever known the joy of riding a bike that's the right size? Test ride both the 19.5 and the 21. If you like the 19.5 better, I agree with Rocco, and you should buy it.

If you like the 21 better but were previously set on the 19.5 because of a sale, limited inventory, buying a particular bike secondhand, etc ... um, I wouldn't do that.

I am 5'11", slim, 32" Inseam and ride a 19.5" frame, tried a 18" frame and felt like a clown on a circus tricycle..

I tried a 21" frame and It was a bit hard to move beneath me and super tight switch backs were wobbly. I felt too stretched over the top to make super slow maneuvers work..

I know a 6'4" long leg rider on a 21" bike, he tried mine,, no go, no way, he could ride a bigger frame I think..

also, the manufacturer's listed frame size is just a starting point. my uncle is 6'2" and rides a size medium yeti sb-95. it's a pretty damn big medium, and fits very similarly to my size large santa cruz blur tr.

but even accounting for variations in geometry, 18" is way small for 6'3".

Hopefully you only like riding gravel or paved trails. Or, like going over the bars...

Too small of a bike usually puts to much mass out front for trail riding.

I'm 6 ft and am comfortable riding a 17 inch Kona MTB. The seatpost has to be extended to its max, but it works for me. If you can ride the bike and its comfortable for you thats what matters.