tshark can convert pcapng file in which all packets were captured through one interface to pcap file. If the pcapng file contains multiple interfaces information, tshark will abort in the middle of conversion:
$ tshark -F pcap -r in.pcap -w out.pcap
tshark: Frame 99 of file "in.pcap" has a network type that differs from the network type of earlier packets, which isn't supported in a "Wireshark/tcpdump/... - pcap" file.
pcapng_to_pcap discards all interfaces information from pcapng file and save the packets as if they had been captured through Ethernet.
Please install PcapPlusPlus before building pcapng_to_pcap. E.g., on macOS:
$ brew install pcapplusplus
Clone and run make, then:
$ pcapng_to_pcap in.pcapng out.pcap