The "Is Empty" format rule for conditional formatting in Proton Sheets is broken. It improperly applies formatting to cells that have non-formula text typed in them (meaning cells that don't start with an equals sign). It also improperly applies formatting to cells that have formulas with errors in them. It should only apply to cells that are actually empty. The only time it properly detects that a cell is not empty is when that cell has a formula in it with a valid (non-error) result.

"Is Not Empty" is also broken in a similar way (it only works correctly when a cell has a formula that has a valid non-error result).