And even lower in a viv that itself is heated. RH is 'relative humidity' -- relative, that is, to the temperature. As the temp goes up, the RH goes down at the exact same level of water in the air.RH in the UK might be 80-85% outside, but it won’t be anywhere near that inside. Heating etc will dry the air out and it’s more likely to be somewhere between 40 and 60 inside the house
In any event, the RH in the viv is almost irrelevant, especially if the probe is "in a sheltered location away from heat lamps" (and as mentioned may well be broken; humidity probes simply cannot tolerate viv conditions for long). An RH meter can be a useful tool, but in no way fully indicates that the level of moisture in the viv is acceptable.
If the frogs' last position is the substrate, they may be trying to drink (the flat position suggests this). A full screen viv with 50w of heat added is almost certainly too dry, especially since the viv does not get thorougly misted (it appears not to, given the position of the nozzles). I suspect the frogs are dehydrating. Do they hide mostly, or come out a lot? Hiding indicates (among many other things) lack of moisture.
The whole viv should get entirely (every surface but the front glass) misted at least once per day, preferably in the AM, and then have enough screen that most of the exposed surfaces -- plant leaves, leaf litter -- fairly dries out over the course of the day (which for almost everyone is a couple inch strip running the width of the viv, along with the under door vent). A viv with a full screen top is almost certainly too dry, unless misting is done every hour or two.