For the record, the tooltip for Builders specifically mentions them having charges, but the tooltip for Military Engineers makes no mention of charges. This is of course subject to change, but it's a semi-reasonable assumption to make that Military Engineers do not use a charge-based system for their build actions. Either they build instantly, or function similar to old workers.
It would actually make sense if they functioned more similarly to old workers, because if I remember the logic for switching to builders with charges that instabuild, the argument was that it was too meticulous to need to plan tile improvements several turns in advance using the build times of workers, because of all the adjacency bonuses involved with improvements and districts. But if roads and forts are not involved in adjacency bonuses (which seems to be the case, haven't seen them utilize this mechanic for them), then it's okay for them to have a build time.
That doesn't necessarily mean they will, of course, and honestly there's probably nothing wrong with military engineers building a road in 1 turn (forts are a different matter, unless there's some kind of upgrade path for forts like in Fall from Heaven 2--build a fort, it starts off weak, but then you can build on it turn after turn to make it progressively stronger).