There is not enough information in the post to determine whether this is seeker behaviour, geometry or a bug. Lock range can change greatly with target aspect, engine temperature, weather/clouds, background contrast and countermeasures; the stat-card maximum is not a guaranteed lock distance in every condition.
Please provide the map/weather, target aircraft, range, front/rear aspect, seeker mode and whether the target was against open sky or terrain. Test the same aircraft in Test Drive from rear and head-on aspects without flares, then attach a short video showing the seeker indicator. If a valid target cannot be acquired under repeatable clear conditions, include the client replay and .clog in a bug report. With those details, other Type 81 (C) users can reproduce the exact case instead of guessing.