Thanks - if it were just Isaac missing, then one could believe odd things happened. But seeing several in one month makes me suspect we are missing something like a garrison chapel. Or a register kept by a chaplain at Hilsea that's been lost.
Frankly I get lost between the different varieties of ways that vital events could be recorded for the Army, partly because the data seems to have ended up in different places and getting a precise description for what is held by the GRO (rather than TNA) seems oddly difficult.
I've just found that there are registers of burials, for instance, at TNA in WO 156 - couldn't find anything (in the era relevant to Isaac) for Hilsea, Portsmouth, Portchester. I note that burial registers for the "Garrison Chapel, Portsmouth" are at TNA for 1849 onwards - nothing earlier - while "Hampshire Burials" on FindMyPast has burials for "Portsmouth Royal Garrison" up to 1841, so it looks like there's something missing there, for starters.
Thanks for your help - I'm a bit wiser if no better informed....