It is correct that the probe on the forward footpad was omitted to avoid interfering with the ladder:
The probe located on the forward landing gear was deleted because of a concern that the failed probe could interfere with crewmen descending the LM ladder. [p. 8]
There was more than one probe for redundancy. In particular, there was no electrical latch for the signal, so if one probe had made contact but then failed, the
CONTACT light would turn off. But if a second probe made contact, the light would stay on.
The probes are hinged in the middle and are designed to bend a particular way upon contact. There's no report of malfunction in the mission reports or in D-6850. However, D-6850 reports that the contact switches were the most common component of the landing gear to fail during manufacturing or testing. Accidentally tripping them was also a common problem while placing the LM inside the Saturn Lunar Adapter:
The item of greatest concern has been the possibility of misuse of landing-gear hardware inside the SLA where the working area is cramped. Several instances of inadvertent actuation of probe switches have occurred on vehicles being readied for launch. In addition, concern for misuse of landing-gear hardware, such as using struts for handholds or footholds, prompted a special training program for personnel working inside the SLA to preclude any further incidents. [p. 52]
One of the final checks on the launch pad was to make sure these probes had not been accidentally actuated.
Source: Apollo Experience Report: Lunar module landing gear subsystem, NASA Tech Note D-6850.