Specifically player created/newly formed B teams seem broken for me in FM26. End of season 1 I created a new B/II team for Bayer Leverkusen since they don't start with one. Game says it will play in the "regional leagues next season". Regionalliga should be 1 division below playable so promotion to playable leagues is possible. Set B Team rep to 9500 with the editor. Resigned and simmed for 10 seasons. The newly formed B Team never gets promoted.
With Leverkusen, player quality in the B team should not be a problem. Checked in the editor and the league ID for Leverkusen II matches Regionalliga. So it's not an issue with the team being in a lower tier unplayable league and the game not promoting/relegating teams between unplayable leagues (I seem to recall this never worked). Also not a problem for B teams that exist in the standard database. St Pauli II got promoted out of Regionalliga for example.
Kind of frustrating for a youth development challenge.