Great question! First, it's worth noting that a Notice of Intent to Lien is not a required document in Texas, and it doesn't officially stake a legal claim. So, because this is merely a tool for recovering payment prior to making some claim, the amount shown on a Notice of Intent to Lien isn't really restricted by any rule or regulation. That being said, only amounts that are actually owed and unpaid should be included in a Notice of Intent to Lien.
Now, to the heart of the question...
Retainage amounts represent amounts that are owed for work that's been performed - even though retainage might not be expected to be paid quite yet. Still, retainage amounts can be included in lien claims since they represent money owed for work done, so if a claimant is going to send a warning or threat of lien, such as a Notice of Intent to Lien, it makes sense that they might include retainage in their Notice of Intent.
Of course, keep in mind that even though retainage might be included in a Notice of Intent to Lien, when a customer finally moves to make payment, they might be reluctant to prematurely release retainage. It makes sense, to some degree - regardless of whether pay apps or invoices were being slow-paid, the plan (if retainage was set to be used on the project) was to withhold some retainage until a later date. And as long as the deadline to make some payment claim, such as a mechanics lien, isn't rapidly approaching - it might be appropriate to continue with the planned retainage practices, at least for the time being. But, if issues persist, claims to recover retainage will be available.
I hope that was helpful, and feel free to return to the Ask an Expert Center with any questions you may have! Below, I've linked some resources I think might be helpful if you have further questions about retainage.