There are many package maintainers who caused F25 packages being newer than F26 packages, so during F25->F26 upgrade, dnf wanted to downgrade them, for example packages quoted below. I think that bodhi quality assurance should be improved to prevent such events that break updates "tree"
expat i686 2.2.0-2.fc26 fedora 91 k expat x86_64 2.2.0-2.fc26 fedora 92 k expat-debuginfo x86_64 2.2.0-2.fc26 fedora-debuginfo 199 k expat-devel x86_64 2.2.0-2.fc26 fedora 62 k httpie noarch 0.9.4-7.fc26 fedora 106 k mariadb x86_64 3:10.1.21-5.fc26 fedora 6.4 M mariadb-common x86_64 3:10.1.21-5.fc26 fedora 68 k mariadb-config x86_64 3:10.1.21-5.fc26 fedora 31 k mariadb-debuginfo x86_64 3:10.1.21-5.fc26 fedora-debuginfo 98 M mariadb-devel x86_64 3:10.1.21-5.fc26 fedora 918 k mariadb-embedded x86_64 3:10.1.21-5.fc26 fedora 4.2 M mariadb-errmsg x86_64 3:10.1.21-5.fc26 fedora 205 k mariadb-libs x86_64 3:10.1.21-5.fc26 fedora 661 k mariadb-server x86_64 3:10.1.21-5.fc26 fedora 18 M mariadb-server-utils x86_64 3:10.1.21-5.fc26 fedora 2.2 M pcre i686 8.40-7.fc26 fedora 203 k pcre x86_64 8.40-7.fc26 fedora 202 k pcre-cpp x86_64 8.40-7.fc26 fedora 40 k pcre-debuginfo x86_64 8.40-7.fc26 fedora-debuginfo 1.2 M pcre-devel x86_64 8.40-7.fc26 fedora 546 k pcre-utf16 x86_64 8.40-7.fc26 fedora 188 k pcre-utf32 x86_64 8.40-7.fc26 fedora 179 k pyp2rpm noarch 3.2.1-3.fc26 fedora 76 k qt5-qtbase x86_64 5.7.1-15.fc26 fedora 3.0 M qt5-qtbase-common noarch 5.7.1-15.fc26 fedora 32 k qt5-qtbase-debuginfo x86_64 5.7.1-15.fc26 fedora-debuginfo 138 M qt5-qtbase-devel x86_64 5.7.1-15.fc26 fedora 3.9 M qt5-qtbase-gui x86_64 5.7.1-15.fc26 fedora 5.2 M qt5-qtbase-mysql x86_64 5.7.1-15.fc26 fedora 60 k system-config-language noarch 3.4.1-1.fc26 fedora 116 k xorg-x11-drv-nouveau x86_64 1:1.0.15-1.fc26 fedora 100 k xorg-x11-drv-nouveau-debuginfo x86_64 1:1.0.15-1.fc26 fedora-debuginfo 402 k + others on other machines, like jetty-schemas libreswan pcre-* pyp2rpm