But in other checkers, all jumps are finished before the promotion happens!

E.g. official draughts rules:
"4.15. Confirming article 3.5., a man who passes during a multiple capture, over a square of his opponent’s promotion row, but ends on another row, remains a man at the end of the capture. "

Or brainking's rules for Draughts:
"However, if a pawn reaches the last row while jumping and can jump more pieces backwards, it does not promote and continues jumping as a pawn."

Turkish, Draughts (International), Canadian, Brazilian, Dameo all work that way - a piece jumps as much as it can and then gets promoted. (While English, Czech, Italian, Spanish / Portuguese, Thai doesn't allow backwards or side jumps, so a piece can never jump further once it reaches the king's row).

Only russian is an exception.

If Gothic is an exception too, I think it needs to be stated more clearly in the rules.

It is very weird to me as a multi-variant checkers players, to see a jump sequence aborted in the middle.

In my Gothic Checkers game ( Gothic Checkers (didero vs. mirari) ), I was expecting to be able to move 17... e5xg3xe1xc1 - but when entering the move, it stops upon reaching the king's row (after e5xg3xe1).

If this is the intended way for Gothic Checkers to work, the rules needs to be made clearer, as this is different from all other checkers variants I am familiar with - generally, either a piece is only crowned at the end of its move (after all possible jumps have been made), or, in the case of Russian checkers, is allowed to immediately continue jumping as a king - I can't think of any other variant where the piece's movement is halted while there are still available jumps to be done as a man.