I'm made a detailed post about this issue on reddit: https://www.reddit.com/r/masseffect/comments/ngmb6g/psa_why_you_are_stuck_at_level_29_or_59_and_why/
I'll go ahead and repost an edited version:
If you look at this Level Progression Spreadsheet Legendary Mode is essentially giving you the equivalent of 2 Classic levels per Legendary level, they started that process at level 2, which means you will always end up on an odd level.
LEGENDARY MODE HAS NO EQUIVILANT FOR LEVEL 60 IN CLASSIC MODE
2=2-3
3=4-5
4=6-7
....
29=56-57
30=58-59)
You can see the problem with this.
I did a lot of testing with the console to figure out what was going on and it seems like if you level 1-29 in Legendary Mode, your xp requirement will cap out at 662100, but you will continue to get xp over this amount up to and be stuck at level 29. This is the amount required to hit level 59 in the original game, as well as Classic Mode. However, you will have earned only 97 total talent points to spend.
Now, if you go into your settings and change it from Legendary to Classic, guess what? Your character now displays as level 57 (should be 59), BUT...your xp will be displaying a number over 662100/542100.
Next, go gain some xp and you will instantly level, and your character will jump from the displayed level 57 to 59, but now your required xp to reach 60 will be 862100. You will also be granted another 2 talent points to spend, bringing your total up to 99.
If you switch back to Legendary mode at this point, the game will remove the xp requirement displayed, and it considers you level 30...despite actually not rewarding the final health increase, or talent point to reach 100 total...because, you are actually still level 29.
One thing that absolutely needs to be addressed is that even if they fix the bug that is preventing people from reaching level 30 or 60, in its current form the level progression for Legendary Mode is fundamentally broken from a mathematical standpoint. Every level from 2-30 is equal to 2 levels each for Classic. That's 29 earnable levels in Legendary Mode, 59 earnable levels in Classic Mode. If each earnable level of Legendary = 2 Classic, thats 29 x 2 = 58. Classic is 59 / 2 = 29.5. These two underlined have to equal out.
Which means, you have to either:
a) Consider the character Level 2 when starting a fresh game when Legendary Mode is enabled.
b) Prevent players from being able to switch between XP modes and lock them in from the start, then recalculate the talent points given either at the begining of the game at level 2, or level 30. (Start at 6 instead of 3, or final level 29-30 grants 3 points instead of 2.