I've been power-leveling my current playthrough of Morrowind. On my last level-up, I got 10 long blade levels (Major skill) and 10 conjuration (Misc). When I went to rest and meditate on what I had learned, I was greeted with only my +5 levelup bonus to intelligence (from conjuration), but that was the only bonus I was given, the 10 long blade levels didn't give me points towards strength. I am certain that I leveled exactly 10-10. I figured it was a strange bug so I reloaded my game and got a quick level in my Axe skill, aon nd it still didn't give me a bonus. So, I just put the +1 on Strength and used the console to add another four levels. Is this a known bug, and why did it happen?

There's a known glitch which affects the 4 left hand Attributes on the level up screen (which includes Strength).
To wit: These Attributes can not reach 100 via a multiplier. (And, as you found out, any time this is attempted the multiplier is reduced to 1).

I suspect your Strength was at 95. If this was the case you could have reached 97, 98, or 99 with a multiplier, but not 100.
In the same vein, if one of these Attributes were, for example, 97, a 3x multiplier would be reduced to 1.

Yes, it was at 95. What a strange glitch. Do you happen to know if it is still possible to get all of your attributes to 100 without circumventing this bug?

The Code Patch fixes this glitch.

... not without introducing a new one: I got some 101's using the code patch.

I was told that this was how the game developers wanted it originally?