It's pretty consistent.

last good:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTLinuxUBSanVptr%20tester/builds/1135
first bad:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTLinuxUBSanVptr%20tester/builds/1136

ChromeContentBrowserClientClearSiteDataTest.Parameters (run #1):
[ RUN      ] ChromeContentBrowserClientClearSiteDataTest.Parameters
../../chrome/browser/browsing_data/browsing_data_remover_factory.cc:49:10: runtime error: downcast of address 0x3951cfcbecf0 which does not point to an object of type 'BrowsingDataRemoverImpl'
0x3951cfcbecf0: note: object is of type 'MockBrowsingDataRemover'
 00 00 00 00  28 91 6b 18 00 00 00 00  b0 91 6b 18 00 00 00 00  00 ed cb cf 51 39 00 00  00 ed cb cf
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'MockBrowsingDataRemover'
    #0 0x9dc7469  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x9dc7469)
    #1 0x9e2e29e  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x9e2e29e)
    #2 0xb07cf2  (/b/s/w/irLXD9XT/out/Release/unit_tests+0xb07cf2)
    #3 0x8eb4320  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x8eb4320)
    #4 0x8eb64bd  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x8eb64bd)
    #5 0x8eb7bb2  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x8eb7bb2)
    #6 0x8ec90e7  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x8ec90e7)
    #7 0x8ec80da  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x8ec80da)
    #8 0x7b3fee1  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x7b3fee1)
    #9 0x7b43782  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x7b43782)
    #10 0x7b435e8  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x7b435e8)
    #11 0x7b30c8f  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x7b30c8f)
    #12 0x7f11000577ec  (/lib/x86_64-linux-gnu/libc.so.6+0x217ec)
    #13 0x84afc8  (/b/s/w/irLXD9XT/out/Release/unit_tests+0x84afc8)


This looks like a legit error.