There is an interesting parallel I've been thinking about regarding Lancelot's absorption of the lake.
We can find a similar mechanic in the light novel series Re:Zero, specifically regarding how "mana poisoning" works.
In Re:Zero, mana poisoning is a lethal condition that affects individuals with flawed mana gates.
The main character, Subaru, suffers from this exact issue after destroying his gate which made it unable to release mana.
To stay alive, he originally had to visit a specialized doctor to regularly siphon away his built up mana.
Eventually, he formed a contract with a spirit who now drains it for him automatically. Without this constant venting, he would die; his body simply cannot process mana like normal people because his broken gate traps his internally generated mana, causing it to build up dangerously without any natural release.
Conversely, another character named Reinhard has a "defective gate" that makes his gate act like a total vacuum. It doesn't just build up his internal mana, but violently sucks in all the ambient mana from the surrounding atmosphere.
By all logic, he should immediately die of severe mana poisoning. However, he was born with a secondary, incredibly rare trait called the "Excess Mana Circulation Constitution."
This condition allows him to safely and continuously circulate that ridiculous volume of mana entirely within his body, transforming him into a an unstoppable powerhouse and the undisputed strongest character in the series.
I believe Lancelot operates similarly to Reinhard in this regard. He essentially possesses his own version of an "Excess Mana Circulation Constitution" because Hazy Moon sets him as the default target for its magic.
As a result, the magic constantly recirculates inside him rather than building up, which is precisely why he doesn't die of mana poisoning like other who tried to drink from the lake.