This seems to be an issue specific to dual-band WiFi routers, which are those that are compatible with both 2.4GHz and 5GHz spectrum bands.
'Under certain conditions, [the] Ipad may not automatically rejoin a known Wi-Fi network after restart or waking from sleep. This can occur with some third-party Wi-Fi routers that are dual-band capable when: Using the same network name for each network, [or] Using different security settings for each network,' Apple wrote."
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