It took me a little while to figure out that the "Select" tool only selects tokens, not walls. Renaming it to "select tokens" or grouping with the token buttons would help.
It's not clear to me what the "radius" of a token is for. At least D&D 5e is played in a non-euclidean space where the diagonal of a 5ft square is the same length as the edges, so a "circle" looks the same as a square. A D&D monster that can attack adjacent spaces can attack all 8 adjacent spaces; a monster with 10ft reach can attack anywhere in the 5×5 square centred on themselves. As a result, your "10ft" radius highlights too few squares, your "15ft" radius highlights too many.
Of course, D&D is not the only game in town, and other systems (or even older versions of D&D) may have different rules you're more familiar with, so this isn't necessarily a "bug" but I figured I should mention it.
This is not generally true. Mechanics in 5e with a radius affect a sphere of that size, and for grids normally the rule is that grid spaces are affected if their center is within the radius. So TFA's token radius is probably meant for ongoing effects centered on the player - light cast by a torch, the range of a paladin aura, etc.
The funky diagonals you're describing are an optional variant rule that's often used to simplify movement, but I've never seen a group that measures spell effects with that rule.
In Pathfinder 2e, for a different example, melee reach works on those non-Euclidean rules, while every other range/area measurement does the "every second diagonal square counts double" rule. So for that, you actually need both versions of radius measurement.
It's not clear to me what the "radius" of a token is for. At least D&D 5e is played in a non-euclidean space where the diagonal of a 5ft square is the same length as the edges, so a "circle" looks the same as a square. A D&D monster that can attack adjacent spaces can attack all 8 adjacent spaces; a monster with 10ft reach can attack anywhere in the 5×5 square centred on themselves. As a result, your "10ft" radius highlights too few squares, your "15ft" radius highlights too many.
Of course, D&D is not the only game in town, and other systems (or even older versions of D&D) may have different rules you're more familiar with, so this isn't necessarily a "bug" but I figured I should mention it.