I made a change to the firmware on the IR sensors a few months ago. This was to resolve a problem whereby when using a highly reflective bed surface (e.g. white or shiny aluminium), some probes gave a false trigger at a great height (usually several cm) as well triggering at the correct height. So the newer sensors don't trigger unless the total received IR is above a certain threshold. I verified that they still work with dark surfaces, including black/grey BuildTak and with glass backed by black paper.
You can tell if your sensors have the new firmware, because at startup it gives one short flash, then a pause of about 4 seconds, then the usual 2 or 4 flashes depending on what mode it is in. The older firmware doesn't give the short flash at the start.