I am having the same issue. I also know of dozens of users on the Zoo Discord that have encountered the exact same issue. You can't use mana generated from Jegentha and several players have lost games because of it.
I wasn't able to use the mana to cast Tinker, but I was able to use it to activate Golos, Tireless Pilgrim's ability. So the bug may only be in regards to casting spells, not activating abilities.
Hello Goodnight. I want to report a bug that has jeganta, it doesn't allow you to pay spells with your ability. This happened to me in a league N° 7022 match N° 254092317 GAME 819458292 and loose because that bug.
The client doesn't let me consume Jegantha's mana to cast any coloured-pip spells. For example, if I try to cast Sliver Overlord, then it doesn't let me use any mana produced by Jegantha