Your source stays session-only
The custom source is held in memory for the current tab and rendered through normal text nodes. KeyVelocity never saves that source. Results are also excluded from local history and personal bests by default; the setup screen offers an explicit option to save aggregate scores and key statistics only.
Paste behavior is deliberately different
Pasting into the source setup field is allowed. Pasting into the typing-answer field invalidates the active run, because pasted output cannot represent typing performance.
When custom practice is useful
Use a job-specific paragraph, vocabulary list, support reply or personal draft when you need familiarity with a particular style. For comparable personal bests, return to the standard test with its fixed local corpus.