if you hold and equip a scrying lens at the same time, and remove one of them, the grid goes back to the normal size. unequipping and equipping it again/ stopping holding going back to holding fixes it.

(Disclaimer: this was said without looking at the code nor trying the pre-release) Perhaps the mod should use vanilla's attribute system and define a custom attribute, rather than doing it based on inventory events (or whatever exactly it's doing right now). If nothing else it would be more extensible. Just make sure 2 lenses can't combine their attribute modifier or I suppose you would end up with the opposite problem of this.
Although come to think of it, combining would kind of makes sense, since they are lenses…

going to test if this is since it is now attribute based

nope it is still bugged

if you take it out of offhand and bring it back it still is bugged. but this also may be a MC issue.
tested with 2 additive speed modifiers on chain helmets when in offhand/helmet. it only applied one but it still kept one working.