Trouble installing Plots version 1+ with other packages

When I update Optim to v0.22.0, Plots will be forced to downgrade to v0.29.9. Then I try to update Plots, it changes nothing. After that, I run following and Optim is forced to downgrade to v0.21.0

(@v1.5) pkg> add Plots@v1.6.0
  Resolving package versions...
Updating `~/.julia/environments/v1.5/Project.toml`
  [e30172f5] ↓ Documenter v0.25.2 ⇒ v0.24.11
  [587475ba] ↓ Flux v0.11.1 ⇒ v0.10.4
  [891a1506] ↑ GaussianProcesses v0.9.0 ⇒ v0.12.1
  [a2cc645c] ↓ GraphPlot v0.4.2 ⇒ v0.3.1
  [ee78f7c6] ↑ Makie v0.10.0 ⇒ v0.11.1
  [429524aa] ↓ Optim v0.22.0 ⇒ v0.21.0
  [91a5bcdd] ↑ Plots v0.29.9 ⇒ v1.6.0
  [e88e6eb3] ↓ Zygote v0.5.5 ⇒ v0.4.20

Which package causes the issue here? Thanks in advance.