Weekly Fable usage % increases on every message even when model is explicitly set to Sonnet 5

Plan: Max (5x)

Environment:
Claude Code, model explicitly set via /model → confirmed output: "Set model to Sonnet 5 (default) and saved as your default for new sessions."

Description:
On a Max plan, with the model explicitly set to Sonnet 5, the Weekly · Fable usage percentage increases with every message I send in the session — even though I never selected or invoked Fable, and the session's own cost breakdown shows only Sonnet 5 activity (no Fable line item).

This is not a background process — the Fable % only moves when I actively send a message, in lockstep with normal session usage.

Evidence:

1. First check: Weekly · Fable = 39%, Weekly · All models = 39%, Current session = 43%.
2. After continuing the same Sonnet 5 session with a few more messages: Weekly · Fable = 40%, Current session = 48%.
3. Session cost breakdown confirms model used = Sonnet 5 only (Input: 8, Output: 1.8k, Cache read: 118.5k, Cache write: 28.6k), cost $0.00.

Expected behavior:
Weekly · Fable usage should remain unchanged when no Fable requests are made in the session.

Actual behavior:
Weekly · Fable usage climbs in parallel with Sonnet 5 session usage, suggesting usage may be misattributed to the Fable weekly quota (or the "Fable" bar is mislabeled/miscalculated) rather than tracked separately.

Note: This appears related to but distinct from #79337 (which involves Fable being rejected with a credits error). In my case Fable was never selected and no error was shown — only the quota bar itself behaves unexpectedly.