Yes, but it’s not a big deal
There are only two spells that are above 5th level and have the
ritual tag; Forbiddance and Drawmij’s Instant Summons
Let’s look at the wording of the Invocation:
You can now inscribe magical rituals in your Book of Shadows. Choose two 1st-level spells that have the ritual tag from any class’s spell list (the two needn’t be from the same list). The spells appear in the book and don’t count against the number of spells you know. With your Book of Shadows in hand, you can cast the chosen spells as rituals. You can’t cast the spells except as rituals, unless you’ve learned them by some other means. You can also cast a warlock spell you know as a ritual if it has the ritual tag.
On your adventures, you can add other ritual spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it.
So you start with two spells from any class list, so long as they are rituals. As you adventure, you can also transcribe other ritual spells, so long as their level is half of your warlock level, rounded down. Not the level of spell you can cast. So at 12th level, you can cast 6th level ritual spells, so long as you have acquired and transcribed them into your Book of Shadows.
Additional, you can “cast a warlock spell you know as a ritual if it has the ritual tag.” But once again, this list is very limited; seven total if we include all Warlock sub-classes:
- Comprehend Languages
- Illusory Script
- Unseen Servant
- Feign Death
- Meld into Stone
- Contact other Plane
And it works for Arcanum, too
If some future rule change adds either Forbiddance or Instant Summons as an Arcanum, or they make a new sub-class with a new 6th-level ritual spell, those work too.
The wording of Arcanum states:
At 11th level, your patron bestows upon you a magical secret called an arcanum. Choose one 6th-level spell from the warlock spell list as this arcanum.
You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again.
At higher levels, you gain more warlock spells of your choice that can be cast in this way.
So Arcanum are warlock spells, and the Invocation states that the character can cast ritual warlock spells as rituals.