Adobe Flash Professional CC (called Adobe Animate CC as of February 2016) is the first 64-bit version of Flash and according to Adobe it had to be re-written from scratch.
Animate CC (versions 2015, 2017, 2018, 2019.0, 2019.1) has at least one new critical bug which required a workaround for the Smart Magnet Rigs to function.
As explained in the article on Smart Magnet Rigging, Symbol instances are being assigned metadata IDs via the SMR Panel. These IDs enable the use of Smart Transform, Smart Magnet Joint, KineFlex and so on.
In Animate CC when you use the default Convert to Keyframe or Insert Keyframe [ F6 ] functionality the metadata is wiped out.
To be able to use Smart Magnet Rigs in Animate CC (versions 2015, 2017, 2018, 2019.0, 2019.1) you need to switch to Convert To Keyframe Advanced completely. The easiest way would be to load the EDAPT Shortcuts or re-assign [ F6 ] and [ Alt+F6 ] to this command.
All older versions of Flash retain the metadata between keys just fine.
The above also applies to the use of Smart Graphic Control panel, introduced with v.4.0 of EDAP Tools.