I am unable to upgrade my heirloom weapon Charmed Ancient Bone Bow with the item level 45 token. The weapon is stuck at item level 44. I am also unable to upgrade the Upgraded Dwarven Hand Cannon; it is stuck at item level 39. And the next token requires item level 40.
These are most likely straggling tooltip errors on the tokens that occurred after the level squish. You should be eligible for the next level token. So you can probably view the 39 item as essentially 40, and the 44 as 45, and upgrade accordingly.
I think this tooltip error was reported back when Shadowlands launched. They lowered the heirloom progression down to 49 from 50 when 60 was max level so you could no longer use them to level from 50-51. That is no longer the case now that you can buy upgrades to max level, and I’m guessing those will work until 59 when another expansion launches.