Seems like a bug this, should be able to set the loop mode on multiple slices at once, like you can set the transpose for example.
==edit==
oops my mistake you can do ithttps://forum.renoise.com/t/added-3-1-b6-quick-way-to-disable-looping-on-many-samples/44736
it’s in the Sample Properties panel, not on the right under the waveform