In an installation predating 2018, a maximum of 64 sensors, actuators, line couplers and backbone couplers can be connected to a TP main line.

True. Before the 2018 update of the KNX TP specification, a main line was limited to a single segment of 64 devices (sensors, actuators, line couplers and backbone couplers combined), with no possibility to extend it via repeaters as a regular line could. After 2018, this restriction was lifted and the modern rule allows up to 256 devices in total per main line, in line with the rest of the topology. Legacy installations therefore still operate under the older 64-device cap.

When you take over a pre-2018 installation and want to add devices on the main line, always check whether you are still under the legacy 64-device cap — pushing past it is a classic 'why is my coupler dropping telegrams' incident.