It is probably fungal, and a very prevalent lawn disease this time of year is Brown Patch. The patches of dead grass blades this causes don't always look like distinct patches, despite the disease name, but multiple lawn fungal diseases can cause brown patches in lawns that look quite similar to each other (for example, Dollar Spot, which also peaks in summer).

No fungicide treatment is needed, nor would it be able to cure any existing infections. The grass should grow out of the damage in autumn when cool-season lawns like tall fescue resume active growth. Yes, overseeding in autumn is a good way to improve the lawn's lushness and resilience to disease by using recommended cultivars (when you can find them) of tall fescue. If needed, when you're ready to overseed, rake away any remaining dead grass matted on the soil surface so the seed gets good contact with the soil for better germination.

Miri