Well, that isn't the issue. Took it out of the case today and tried it with the PCB out of the case and it replicates the issue.
Been playing around some more and the exact issues are.
Hold down Left Shift and quickly press M then O, also get Q.
Hold down Left Shift and quickly press M then K, also get A.
Hold down Right Shift and quickly press M then O, also get }.
Hold down Right Shift and quickly press M then K, also get Enter press.
I never use right shift so luckily I don't have to worry about the Enter press, which is probably the only one of the 4 issues that would cause me actual trouble, depending on the situation.
I guess one important thing to note is that two of the pads didn't work. Not lifted, just didn't work out of box. I tested the entire PCB as per Hasu's instructions and every key did work. However, the two pads circled below didn't work once I soldered in the switch. Those pads are the ones that connect to the diode, if I'm not mistaken. I tested it again after soldering by touching the right pad (the one not circled) and the square pad of the diode, as shown in Hasu's instructions, and it would still work. However, pressing the switch down, did not.
I used some wire to fix that, like so (tbh I actually forgot if that's how I did it but I'm about 90% sure it is):
Idk if any of these things are contributing to it. I don't exactly care enough to make a big deal of it or continue wasting too much time on it, since it's not that big of an issue. I'll try reflashing firmware, later, in case that could be the cause.