You (re-)invent the spell even when taught by a teacher. So a lab text will be created in that case too.
Even when you learn a spell from a teacher,
it is your Wizard’s Sigil that manifests in your
version; you actually invent the spell yourself,
but with constant guidance from the teacher.
Learning Spells from a Teacher is a still lab activity. It allows the magus to create new effects (invent spells). Ergo, lab texts for those spells are created.
Spells give similar spell bonus for enchanting.
A spell known by the enchanter gives similar spell bonus to enchanting that effect into an item. A spell lab text, however, does not provide any benefit to enchanting a similar effect into an item.
Wax tablets are easily erasable/reusable. I can see a parens getting their apprentice to create lab texts on such "cheap" media.
I have previously considered lab texts to be more permanent. I may have to think about the ramifications of transitory texts.
How about alchemical texts then?
I do have to clarify mages definitely transfer the notes to the parchments in their script during the end of day, and the end of week, just like everyone else did. But it means the apprentice does not get those laboratory texts.
Honestly, that sounds like exactly the sort of task that my mage would use an apprentice for. It is really weird to me that Ars Magica does not consider 'Copying a Book' to be anything like 'Reading a Book'. Require the apprentice to carefully copy notes from magical lab work, answer questions whenever they find something they do not understand, done... but it is not that way in RAW.
I thought that, but overruled it as the apprentices cannot read the shorthand of their parens. If the apprentice would enscribe them, the apprentice would know the script.
It is mentioned that if an apprentice has helped the Magus in the lab, he could read the shorthand of that specific lab text
Agreed. Keeping the lab notes may actually be part of the tasks that the master delegates to the apprentice. That frees up time for the master to do more, or not interrupt himself to take those notes during an experiment, partially explaining the bonus from being assisted in the lab.