Did you build yourself? The HAVE_BROKEN_RTC build option should be disabled by default in official builds. LuCI currently expects UNIX timestamps in the expiration time column and not relative remaining validity times.
I am considering adding some heuristics to workaround the problem. Basically if the expire value is smaller than a specific cutoff date (say 2020-01-01, 1577833200) then treat it as relative, else as absolute expiration value, then act accordingly.
Handling the other mode seems quite difficult, as the first column will only statically show the lease time. I am actually not sure if there is a way to calculate the remaining time, unless it is possible to find out when the lease was handed out and calculate from there.