If you review my posts over the years, I have consistently castigated TSA as an agency and many TSOs for their irrational, unacceptable, often criminal policies, procedures, and behaviors, using the strongest negative language permitted on the FT board (and I use far stronger language in person). So if this comes off as a defense of TSA, just keep that in mind.

I think it's reasonable and expected that PreCheck has limited hours, compared to the regular screening lines.

Why do I think so?
1) TSA did not create PreCheck to make things more convenient on pax; they created it to make their own job easier by reducing the number of people getting the full monty of screening, and to keep the lines moving faster by giving frequent travelers a separate line where they don't have to wait behind the 'kettles'.
2) During off-hours (i.e. before 5am or after 8pm), traffic through the c/p is generally lower, and even if you get the full monty, your wait is less than if you got the full monty during a busy time. So it's smart to reduce the overall number of open lines, and if that means consolidating the PreCheck lines into the regular lines, I don't think that's unreasonable.
3) Not every airport is the same. Some have more TSA staffing than others, some have better TSA management than others, and some have better TSA attitudes toward their jobs and toward pax than others. You cannot expect exactly the same service even at the same airport from day to day due to differences in management styles and staffing levels, let alone at different airports on different days at different times.
4) Forgive me for being the told-you-so type, but PreCheck has always been a program I have held in some contempt - you pay for the privilege of allowing TSA to pry into your background, while being told that it's such a great product, yet all the while every piece of printed material about PreCheck constantly reminds you that you are never, ever guaranteed to get the PreCheck benefits at any given airport, c/p, time of year, day of week, or time of day. TSA has essentially run a con by selling something that you may or may not get, that you may or may not like when you do get it, but all you're really getting is a lottery ticket to maybe not have your dignity, rights, and freedoms stripped away from you in the name of some unattainable safety, just because you want to get on a plane.

All of that being said, again, I don't think it's unreasonable for TSA to limit the hours that the PreCheck line is open, to manage their staffing levels during off-hours when overall traffic through the c/p is lower. I'm sorry that people have paid for something and don't feel that they're getting value for their $15.60 a year if they don't find the PreCheck hours convenient, but I'd rather the program be limited in scope than to have the TSA budget bloat even more than it already is to keep PreCheck lines open at all times in every c/p in the country.