Two instances of Route 25A have been sighted, though I've seen nothing official about the second one.
In 1932 (and possibly 1933), Route 25A existed along Hawleyville Road in Newtown, from Currituck Road south to US 6. Route 25 followed Currituck Road into Newtown Borough at the time. By 1934, Route 25 was rerouted to Hawleyville Road and took over Route 25A. The route does not appear in the DOT's spreadsheet of route changes since 1933.
Hawleyville Road was built around 1915 by local entrepreneur William Upham, to provide access to the south from the railroad and his businesses (which included a tea shop and peanut butter factory.)
A Trumbull Historical Society page states that after the Route 25 freeway opened (in 1982), the former Route 25 from Route 25/111 to Route 15 was called Route 25A for a while. I have seen nothing in maps or state highway logs about this route; it might have been a local name rather than a state designation.