It's possible that your aircraft has picked up icing which is not currently visible on the frame. This affects the climb performance of the aircraft. I had this same issue. You can check the amount of ice on the aircraft frame from the developer bar.
Lean the mixture until you get the highest EGT, then richen back up by about 2 notches on the gauge. There is also a bindable mixture setting in MSFS that will set the best mixture. You can bind that to a key to give you a guide of where the best mixture setting is. Just note that best mixture setting changes depending on altitude, engine and prop rpm and temp.
I like the Garmin 100, I fly VFR anyway, and that little unit has a wealth of information on it. Winds, temps, different altitudes (MSL, DALT) and Ground speed and others. Very handy.
I KNOW the GNS's do it all too, but the Garmin 100 is not there just for show, it has a purpose.
This is a long shot but if you have previously installed the pms50-gns530 mod from pimarc on github you may need an update to make it compatible with Microsoft Flight Simulator Sim Update 3.
@ZenIT_SWE No idea how many positions other aircraft have, I just use my TCA Axis to handle the flaps but with the PA-28R I have lots of issues and have a hard time using toggle switches and buttons. Flaps work with Up and Down but not with Axis. Gear Up and Down also work but no Toggle else the gear will do some weird stuff.
And there are issues with the way MSFS handles hardware buttons (On/Off) that keeps sending inputs.
@Craig Thanks greatly for the response and my apologies for posting in the wrong place. If I find anything else I will submit bug reports/tickets. I'm really loving this plane though.