Arelith

As it stands, Spell Mantles will read multiple instances of damage from spells such as Flame Arrow and Darkbolt as separate spells, for the purpose of absorbing those spells. This is problematic as Dark Bolt is only a 5th lvl spell, while Greater Spell Mantle is either 8 or 9 depending on class.
The problem therein, is even upon a maximum roll, a Greater Spell Mantle can absorb up to 22 levels of spells.
A Darkbolt, cast by a level 10 spellcaster (The level most spellcasters get access to 5th lvl spells) can immediately break a spell mantle, counting as 5 levels per bolt, at 25 spell levels total, and up to level 14 (the level where Darkbolt hits max scaling), it will not only guarantee breaking the GSM, but will also apply damage due to counting as 35 levels of spells.
As far as resources go, this makes the Spell Mantles entirely worthless as long as they are recognized, given their rather unique VFX, this is easy to spot.
Ultimately, I feel that either Spell Mantles deserve a buff to make them worthwhile (or at least worth the spell component cost)
Or a code needs to be implemented to prevent multiple bolts from the same spell from counting as separate spell levels.

One lesser spell breach will crack it, too. Its nothing new, and I've always considered spells like flame arrow to be direct counters to spell mantle, myself

Flame Arrow is the mantle breaker for this very reason. It's basically been the standard spell to pop a mantle from when NWN came out to now.

Forcing someone to waste an action to pop your mantle doesn't seem useless. Especially when, as others have pointed out, a breach would just pop it anyway.

It's not a bug, it's a feature and a staple of PvP dynamics.
Spell mantles aren't supposed to counter all spells (some spells even fully ignore spell mantles by design).
IMO this applies twofold now that their duration is turn/lvl.

If you had globe of invulnerability active on yourself as well, would it soak spells before the spell mantle?