Is this a glitch in ME1?

Not a glitch.

You do not get any XP for killing the Rachni that are attacking the marines at the listening post. You may get XP for surviving the encounter, but I do not remember (I do not think you do).

At Pinnacle Station, you do not get any XP for 'killing' individual enemies. You get XP for getting first place on each mission (but only the first time you do it) and for completing Ahern's special mission.

Nope, it's a witch/bitch in a ditch. 🤣

Joking aside, it's what the other guy said.

I got to level sixty just before I had to get saren to blow his brains out. I really hate having to redo all my saves. I did the extreme power gamer thing so I could get experience points somewhat faster in my last two playthroughs which are the ones that will romance kaiden in ME1 and my male shep that romances liara in ME1 and ME3

that is what I learned as well.

From what I recall on the old forums -- and so what I keep repeating on these forums -- is that killing with the Mako reduces XP by 50%.

It seems to make the biggest difference early game when the XP required for early level increases are at their smallest. That being said...

I have both Pinnacle Station and Bring Down the Sky DLC. With all XP bonuses from achievements active (which I think is +20% or +25% XP), I was able to hit level 58 by the end of the game in a single playthrough by not killing anything in the Mako. The last few runs where I decided to use the Mako -- at least early game when it was easier -- I hit level 56 by the end of the game.

So I think the Mako XP punishment is a little overblown. Even without DLC, you should be able to hit level 56 or 57 without the Mako, and level 54-55 with the Mako, provided you do everything. Not sure how much those extra 2-3 talent points will help you, depending on your build. You might just be putting those talent points into a point sink at that point.

I hate this so much.

"Hey, here's a cool tank you get to use! But we're going to punish you for using it."

Ah well, honestly, I never really cared about it much.
Whether I end ME1 at level 52, 55 or 58, it doesn't really make much difference (who cares if I can get 1-2 extra points right at the start of ME2 or a little later). You get to lvl 30 in ME2 easily before the end anyway, so I never get out of the MAKO just for XP.

I never liked the "take 'em down to sliver" technique really.  It disrupts my rhythm and  I actually prefer to park the mako in a spot with cover and just work my way in on foot... probably why it never bothered me at all that the nomad had no cannon.