The issue was that because both them and the player had MF_SOLID, the tmfloorz of the spring was getting set to above the player (or vicea versa with tmceilingz), forcing it upwards with them under certain circumstances.
Now, springs only acknowledge the solidity (for purpose of tmfloorz/tmceilingz) of objects they CAN'T launch.
Test with <root>/toaster/srb2springsagain.exe and <root>/DrTapeworm/springtest.wad, via Mystic's secret server that SOME jerk keeps leaking from.