Field group horizontal tabs are displayed as accordions in small screens
Perhaps try out the patch to make this setting configurable as proposed for field_group module in this issue

Users have reported that it applies cleanly and allows for the setting to trigger based on a custom breakpoint.

Testing it out and reporting success/failure on that issue gives a better chance for this feature to be incrorporated into the module itself.

NOTE: the patch probably needs to be applied to the dev version of the field_group module